1. Field of the Invention
The present invention relates generally to honeycomb labyrinth seals used in steam turbines and, more specifically, to an improved moisture drainage provided for honeycomb seals.
2. Description of the Related Art
A honeycomb labyrinth seal used in conjunction with rotatable steam turbine blades is described in U.S. Pat. No. 4,416,457, issued to McGinnis et al. FIGS. 1 and 2 of the aforesaid patent have been reproduced herein in order to illustrate the prior art.
Referring to FIGS. 1 and 2, a tip of a rotatable steam turbine blade 1 is disposed adjacent a honeycomb labyrinth seal 3 which fits into a circumferential groove 4 in an inner cylinder or blade ring 5. The seal 3 is formed as a plurality of arcuate segments. Each segment includes a base portion 7 and a plurality of rows of honeycomb cells 9 extending radially inwardly from the base portion so that each cell is open adjacent the blades 1. A plurality of passages or grooves are so disposed that each cell 9 is connected to at least one passage.
The seal illustrated in FIGS. 1 and 2 provides a sealing device which when located at a rotating blade tip serves as a collector for liquid droplets discharged from the rotating blade through centrifugal action and permits drainage through the grooves 11 to a gap 13 disposed between circumferentially adjacent honeycomb labyrinth seal segments. The water then moves to a drain. Drainage of the liquid is important in order to minimize the possibility of reentrainment in the path of the rotating blades in order to minimize the possibility of blade tip erosion resulting from reentrainment. However, the drainage flow is substantially in a downstream direction resulting from axial pressure gradient imposed by the blade passage flow field conditions.
Low pressure turbines are designed to incorporate as many features as possible which lead to reduced erosion. The entire turbine geometry exposed to steam flow has an effect on moisture collection and removal. For example, one feature is to increase the axial spacing between a stationary row and the next adjacent rotating row.
While various features have been implemented, there is still room for improvement. For example, water accumulated in the honeycomb seal which finally leaves the last row of honeycomb cells may dribble back into the blade path, meaning that a significant fraction of the collected water will become reentrained.
While it has been considered to shorten the length of the honeycomb on the downstream side so that the rotating blade extends beyond the honeycomb, this would result in significant efficiency losses because of the pressure difference between the pressure and suction sides of the blade.
FIG. 3 shows an enlarged sectional view of a known turbine showing a stationary blade of the L-OC row, a rotating blade of the L-IR row, and a stationary blade of the L-IC row. Prior attempts to collect moisture have focused on ways to have water slung off the trailing edge of the L-1R rotating blade pass unimpeded into a moisture drainage cavity 2. However, these attempts have not been completely successful, partially because the drain path is somewhat convoluted.